I am probably doing it all wrong but this is what I do.
Indoor tank maintained at 12c being fed 6 to 8 times a day small amount of wheatgerm or all season, as soon as I open the door they are looking for food. Will bring temp up to 16c in early March
Outdoors unheated not been fed much for a few weeks, always feed a small amount of wheatgerm (mornings only) if the koi are looking for food and temp is 8c or higher.
Have noticed the koi are always looking for food just before a cold spell and they are good weather forecasters.
I believe if the koi are actively looking for food they need some as they will be expending energy and body mass looking for food .